Simple ApplicationController stub for RESTful API and usage example

ApplicationController.rb

class ApplicationController < ActionController::Base
  include ApplicationHelper

  before_action :current_user_session, :current_user

  rescue_from 'StandardError', 'Exception', 'ActiveRecord::RecordInvalid',
              'ActionController::RoutingError', with: :exception_handler

  private

  def exception_handler(exception)
    case exception
      when ActiveRecord::RecordInvalid
        status = :bad_request
      when ActionController::RoutingError
        status = :bad_request
      else
        status = :internal_server_error
    end
    render_response(nil, meta_message(exception.message), status)
  end

  def require_user
    unless current_user
      render_unauthorized_response(nil, meta_message('You have to be logged in to preform this action'))
      return false
    end
    check_permissions
  end

  def require_no_user
    if current_user
      render_unauthorized_response(nil, meta_message('You have to be logged out to preform this action'))
    end
  end

  def current_user_session
    return @current_user_session if defined?(@current_user_session)
    @current_user_session = UserSession.find
  end

  def current_user
    return @current_user if defined?(@current_user)
    @current_user = current_user_session && current_user_session.user
  end

  // ...
end

ApplicationHelper.rb

module ApplicationHelper
  def render_response(object, meta, code)
    response_data = {data: object, meta: meta}
    respond_to do |format|
      format.json { render :json => response_data, status: code }
      format.all { render :text => 'Only JSON format is supported at the moment.' }
    end
  end

  def render_ok_response(object, meta = {})
    render_response(object, meta, :ok)
  end

  def render_created_response(object, meta = {})
    render_response(object, meta, :created)
  end

  def render_bad_request_response(object, meta = {})
    render_response(object, meta, :bad_request)
  end

  def render_unauthorized_response(object, meta = {})
    render_response(object, meta, :unauthorized)
  end

  def render_server_error_response(object, meta = {})
    render_response(object, meta, :internal_server_error)
  end

  def meta_message(message)
    {message: message}
  end

  def meta_created_message(clazz)
    meta_message(clazz.name + ' successfully created')
  end

  def meta_updated_message(clazz)
    meta_message(clazz.name + ' successfully updated')
  end

  def meta_destroyed_message(clazz)
    meta_message(clazz.name + ' successfully destroyed')
  end

  def meta_no_entity_message(clazz, object_id, parent = nil)
    message = 'No ' + clazz.name + ' with id ' + object_id
    message += ' for ' + parent.class.name + ' with id ' + parent.id unless parent.nil?
    meta_message(message)
  end

end

EntityController.rb

class EntityController < ApplicationController
  before_action :require_user
  before_action :get_entity, :only => [:show]

  def index
    render_ok_response(Entity.all)
  end

  def show
    render_ok_response(@entity)
  end
  
  // ...
  
  private

  def get_entity
    @entity = Entity.find(params[:id])
    if @entity.nil?
      render_bad_request_response(@entity, meta_no_entity_message(Entity, params[:id]))
    end
  end
  
  // ...
end